Not Applicable
Not Applicable
1. Field of Invention
This invention relates to a method of preparing iridium crucibles for use in crystal growth. More specifically it relates to a method of surface conditioning by way of weld melting, or weld puddling, such as by a pulsating welder, such as a GTAW welder, TIG welder, laser welder or the like, for the purpose of reducing grain boundary size in the crucible which lead to leakage, cracking and a reduction in the useable life of the iridium crucible during production of crystals such as the YAG laser crystal and scintillation crystals, including Lutetium Oxyorthosilicate (LSO) crystals.
2. Description of the Related Art
Devices for detecting the distribution of gamma rays transmitted or emitted through objects to study the compositions or functions of the objects are well known to the art. The techniques referred to as Emission Computed Tomography can be divided into two specific classes; Single Photon Emission Computed Tomography (SPECT) uses radiotracers which emit gamma rays but do not emit positrons and Positron Emission Tomography (PET) which uses radiotracers that emit positrons. In either discipline, scintillation crystals are utilized to detect the emitted gamma rays. One such scintillation crystal is composed of Lutetium Oxyorthosilicate (LSO). Scintillation crystals, such as LSO crystals, and other crystals, such as the YAG laser crystal, are known to be grown using the Czochralski technique. Simply stated and with reference to LSO, in this technique, LSO is melted in an iridium crucible. An LSO seed crystal is inserted in the molten LSO and the new crystal is pulled from the molten LSO. After pulling the crystal, the residual LSO cools and hardens in the iridium crucible. As the residual LSO cools, it expands thereby placing stress on the iridium crucible. It is known that the re-freezing of the molten LSO causes the sidewalls to swell or bulge outward under the pressure of the expanding LSO. The hardened LSO must then be removed from the iridium crucible. Then, the iridium crucible is inspected for cracking and repaired as necessary. Those skilled in the art recognize that repeated use of the iridium crucible causes grain boundaries to become larger and more distinct, resulting in increased risk of molten lutetium leaking through the enlarged grain boundaries. Further, growth of the grain boundaries often leads to cracking of the iridium crucible. Those skilled in the art recognize that growth of grain boundaries is a problem with wrought iridium crucibles as well as electroformed iridium crucibles. It will also be recognized that iridium crucibles are exposed to the same or similar stresses when used in conjunction with growth of other crystals.
Initially, leakage and cracking were repaired, as necessary, by a process of welding or weld melting the affected area on either the exterior or the interior of the used crucible. In this regard, it has been found that weld melting melts grains of iridium into smaller grains simultaneously repairing the defect and reducing grain boundary size. This led to the observation and discovery that welding reduced propagation of the grain boundaries inasmuch as it was observed that grain boundaries did not readily grow into the welded area. Those skilled in the art will recognize that iridium crucibles are expensive, that repair time takes the iridium crucible out of LSO crystal production, and that growth of grain boundaries shortens the useful life span of an iridium crucible.
Accordingly, there is a need for a method of conditioning the surface of iridium crucibles by weld melting the surface as either a form of preventive maintenance or repair to reduce the propagation of growth boundaries and thereby reduce the amount of time spent repairing an iridium crucible and further increase the useful life span of an iridium crucible.
It is therefore an object of the present invention to provide a method of preparing iridium crucibles for use in growing crystals by conditioning the surface of the bottom of the crucible, the surface of the sides of the crucible or both by way of weld melting, or puddling, the surface such as by a pulse welder.
It is another object of the present invention to provide a method of repairing iridium crucibles by conditioning the surface of the bottom of the crucible, the surface of the sides of the crucible or both by way of weld melting, or puddling, the surface such as by a pulse welder.
Other objects and advantages over the prior art will become apparent to those skilled in the art upon reading the detailed description together with the drawings as described as follows.
In accordance with the teachings of the present invention an iridium crucible is welded on either the entire surface of the bottom, the surface of the sidewalls from the bottom up to a height sufficient to include the depth of the residual molten crystal material after pulling of a crystal, or both the surfaces of the bottom and sidewalls. In this regard, the iridium crucible is pulse welded so as to puddle a small portion of the surface without substantially penetrating the side or bottom wall. It will be understood that the terms weld melting and weld puddling are used to describe the process of pulse welding the surface without substantially penetrating the surface. In the preferred embodiment, the bottom of the iridium crucible is conditioned by way of weld puddling such as by a pulsating welder. Similarly, the sidewalls are circumscribed by a pattern of pulsed weld puddling in a helical fashion. It will be recognized that other patterns of pulsed weld puddling are possible so long as a substantial portion of the bottom and the sidewalls are weld melted as described herein. Further it will be recognized that any state of the art welding apparatus, such as a GTAW welder or laser welder, that generates temperatures sufficient to melt iridium while controlling the depth of penetration can be utilized.